The deposition of GaAs by solution growth (SG), similar to liquid phase epitaxy (LPE), has several advantages for fabricating large area electronic quality GaAs layers on Si substrates. The first of these advantages and the most critical is the resulting high material quality. GaAs material grown from solution on GaAs substrates has, in general, demonstrated devices that are "superior in performance to those grown by other methods", as reported by R. L. Moon, Crystal Growth, 2nd ed., B. R. Pamplin, ed., Pergamon Press, p. 421 (1980). For GaAs solar cells, this superior device performance is seen in high short circuit currents. Using LPE, Hovel and Woodall (H. J. Hovel and J. M. Woodall, 12th IEEE PVSC, 945 (1976)) have achieved the highest short circuit currents for GaAs solar cells. The high currents are a result of long minority carrier diffusion lengths in the SG GaAs.
GaAs growth on Si has been widely studied but solution growth has gone largely uninvestigated. The only known report of the solution growth of GaAs on Si is V. A. Presnov, et al., Sov. Phys. Crystallogr., 23(1), 121, (1978). Until now, solution growth has gone largely uninvestigated for fabricating large area heterostructures such as GaAs on Si. Previous attempts to grow GaAs on Si by liquid phase epitaxy have not been successful. The failures have been attributed to dissolution of the substrate due to solubility mismatch between the substrate and the growth solvent and to lattice mismatch between the substrate and heteroepitaxial layer. Applicants have further discovered that mismatches in thermal expansion, as well as the need to prepare the substrate surface prior to solution growth of the heteroepitaxial layer are also obstacles to successful utilization of solution growth processes for heteroepitaxy.
The work of other investigators for large area GaAs on Si structures has focused on vapor phase growth - chemical vapor deposition (CVD) and metallorganic chemical vapor deposition (MOCVD). GaAs has also been successfully deposited on Si by molecular beam epitaxy (MBE) (for example, Newmann et al., J. Appl. Phys.,61(3), 1023 (1987)) but this process is not well suited to large area deposition or high throughput production. The vapor phase techniques have been successfully used to deposit GaAs directly on Si (M. Shimizu, T. Mizuki, M. Miyago, T. Hisamatsu, M. Enatsu, T. Yamaguchi, K. Sugawara, T. Sakurai, K. Awane, 18th IEEE PVSC, Las Vegas, Nev., 1727 (1985)), on Ge substrates, and on Ge layers on Si (Ge/Si) substrates (M. Kato, K. Mitsui. K. Mizuguchi, N. Hayafuji, S. Ochi, Y. Yukimoto, T. Murotani and K. Fujikawa, 18th IEEE PVSC, Las Vegas, 14 (1985), and B-Y Tsaur, John C.C. Fan, G.W. Turner, B.D. King, R.W. McClelland, and G. M. Metze, 17th IEEE PVSC, Orlando, Fla., 440 (1984)). However, the performance of solar cells made from this material has been limited by high crystal defect densities. Table 1 summarizes the solar cell performance degradation caused by depositing GaAs on Ge substrates and Ge layers on Si (Ge/Si) substrates. For comparison, the performance of several GaAs on GaAs solar cells is also included in Table 1. A solar cell from each material system is included from Kato et al. to compare devices from the same deposition system.
TABLE 1 ______________________________________ Investigator J.sub.sc n = effi- & Growth Area V.sub.oc (mA/ ciency Technique (cm.sup.2) (volts) cm.sup.2) FF .about. ______________________________________ (a) GaAs on GaAs M. Kato 1.0 1.01 31.5 0.84 19.7 (AMO) MOCVD J. G. Werthen.sup.1 4.0 1.05 32.3 0.84 21.5 (AMO) MOCVD H. J. Hovel 0.1 1.015 33.1 0.745 18.5 (AMO) LPE (b) GaAs on Ge M. Kato 1.0 1.18 29.6 0.72 18.6 (AMO) MOCVD (c) GaAs on Ge/Si M. Kato 0.25 0.66 18.4 0.61 5.5 (AMO) MOCVD B-Y, Tsaur 0.093 0.8 23.0 0.75 14 (AM1) CVD 0.51 0.79 23.0 0.61 11 (AM1) ______________________________________ .sup.1 J. G. Werthen, G. F. Virshup, C. W. Ford, C. R. Lewis and H. C. Hamaker, 18th IEEE PVSC, Las Vegas, Nev., 300 (1985).
In Table 1 all three parameters--open circuit voltage, short circuit current, and fill factor--are degraded in the GaAs solar cells on Ge and Ge/Si. (The one exception is the open circuit voltage in the GaAs on Ge solar cell. According to Kato et al., this increase is a result of the photovoltaic effect at the Ge/GaAs heterojunction.) This decreased performance results from the increased defect density in the GaAs. These defects result from strain created by the 4% lattice mismatch between GaAs and Si. These defects reduce the minority carrier diffusion length in the GaAs and cause a corresponding decrease in the short circuit current. Tsaur et al. were successful in reducing the current loss by using growth interrupts to cause the defects to bend over and self-terminate. The short circuit current they report (shown in Table 1), without an AlGaAs window layer, shows that current losses can be minimized. The solar cells of the others contain an AlGaAs window.
The reduction in open circuit voltage is caused by the presence of defects at the GaAs n/p junction and in the bulk that increase the reverse leakage current (J.sub.0). To improve the open circuit voltage the defect density must be reduced.
The reduction in fill factor is largely a result of series resistance at the Ge/GaAs interface and shunt effects in the bulk. This series resistance can be reduced by using high doping levels on both sides of the Ge/GaAs interface. This highly doped Ge/GaAs junction should have low series resistance. A second GaAs layer of lower doping is necessary for the absorber/generator of the GaAs solar cell. The high quality of solution grow GaAs should also reduce shunt effects.